The present invention relates to a frequency control circuit for controlling the frequency of a local oscillator.
More particularly, the invention addresses frequency control problems encountered in a mobile station in a spread-spectrum communication system. The local oscillator in the mobile station generates a local carrier signal that is used in demodulating a signal received from a base station. Accurate demodulation requires that the frequency of the local carrier signal closely match the carrier frequency of the received signal, but size and cost constraints prevent the use of a highly stable local oscillator, and in any case, the motion of the mobile station may introduce a variable Doppler frequency offset. The frequency offset between the received carrier signal and locally generated carrier signal must therefore be constantly monitored, and the frequency of the local must be controlled so as to keep the offset near zero.
One way to carry out the necessary frequency control is to have the base station transmit a pilot signal with a constant data value, providing the mobile station with a known reference against which the carrier frequency offset can be measured. In a Technical Report of the Institute of Electronics, Information and Communication Engineers of Japan (RCS95-105, pp. 34-50, November 1966), the inventors have proposed a frequency control circuit with a multipath frequency offset detector, which operates robustly, even under multipath fading conditions, by detecting frequency offset on each of a diversity of paths on which the pilot signal may be received.
Instead of devoting one communication channel to a pilot signal, however, a spread-spectrum mobile communication system may insert symbols with predetermined values at predetermined intervals in the transmitted data stream, to provide reference data for frequency control. Typically, these reference symbols are inserted at intervals of about one two-thousandth of a second (0.5 ms), that is, at a frequency of 1/(0.5 ms) or two kilohertz (2 kHz). If only the reference symbols are used, the sampling theorem requires that any frequency offset be limited to a range of one-half of 2 kHz, or 1 kHz; frequency offsets outside this range are in principle uncorrectable.
Spread-spectrum mobile communication systems are typically assigned carrier frequencies of about two gigahertz (2 GHz) in order to limit frequency offset to 1 kHz, and the local oscillator of a mobile station must be stable to within one-half of one part per million (0.5 ppm). This requirement is difficult to meet within the above-mentioned size and cost constraints.